Hydraulic analysis of the GEOREAL fluid injection experiment at the KTB pilot hole, Germany

A total of 600 m 3 of fresh water was injected into the 4 km-deep KTB pilot borehole near Windischeschenbach, Germany, in November 2023. The main goal of the GEOREAL hydraulic stimulation experiment was to stimulate fractured metamorphic rocks in the vicinity of the well-characterised SE2 fault zone at 4 km depth with high flow rates and to compare rock and fluid responses to a previous one-year-long injection experiment in 2004/5 into the same formation. The intention was to quantify the potential for heat extraction at a site where temperature and pressure conditions are representative for crystalline rocks for large parts of Germany. Due to an unforeseen incident during packer deployment, however, a reduced fluid volume of 600 m 3 was injected, pressurising the entire pilot hole (KTB-VB), including the cased section to 3.85 km depth and the 150 m open borehole section below. Flow rates were variable ranging from 10 to 220 l/min. Pressure records were obtained at the well heads of KTB-VB and the main borehole (KTB-HB) at 200 m lateral distance at the surface which was also used for downhole microseismic monitoring. Hydraulic data were analysed and compared to results from 2004/5. The hydraulic parameters show similar values as in 2004/5 when no packer was blocking the borehole. Due to an unknown leak in the casing cement of the 35-year-old KTB-VB, the GEOREAL experiment had to be stopped earlier than planned. No microseismic events were detected during the injection using conventional event-detection methods. Hydraulic results for the KTB-VB injections two decades apart suggest that no significant permeability changes have occurred in the SE2 fault zone indicating that fracture healing does not play a major role in the reservoir. The pressure recordings from the KTB-HB showed no change caused by the injection into the KTB-VB, suggesting that either the injected volume was too small to cause a hydraulic response, too much fluid leaked into the formation, the hydraulic connection observed previously ceased to exist, its geometry is unfavourable for this configuration, or hydraulic diffusivity is smaller than suggested for previous experiments.